EDIT: Since this list is rapidly growing, I'm going to actually specify my ground rules for these items:
It cannot be an equipped item, unless it is a Shirt or Tabard
It cannot be something used every day by raiders (hint: dps potions, main stat flasks, healing potions, far too obvious)
If you normally have them in your bags, it's probably not for this list.
I'm working on a list of useful items for world PvP, and figured I'd post what I have so far:
Ashran Only:
Star Root Tuber Instant healing and 40% damage reduction for 10s. Unique-20
Song Flower 15% increase to all stats, persists through death. Unique-10
Preserved Discombobulator Ray Dismounts the target and reduces their damage and healing dealt by 30% for 5 seconds.
Phantom Potion 30% increased movement speed and invisibility for 30 seconds.
Vintage Free Action Potion Immunity to stuns and slows for 15 seconds. Does not break existing effects.
Draenor Only
Inspiring Battle Standard A/H 30% increased maximum health for 1 min or until banner dies.
Fearsome Battle Standard A/H 5k damage to targets within 8 yards of banner every 3 secs for 1 min or until banner dies.
Pandaria Only:
Cursed Swabby Helmet
Eternal Warrior's Sigil Doesn't work above level 94.
Windfeather Plume
Cauterizing Core (20 charges, 1 hr cooldown, soulstone/ankh)
Timeless Isle Only:
Golden Glider (Max 20, Engineeringless Goblin Glider)
Sticky Silkworm Goo (Max 10, short range root)
Black Ash (90% healing reduction, short 20 yard range)
Ash-Covered Horn Super fast 20 second duration land mount
General Purpose:
Aqua Jewel
Hardened Shell Does not work at 100
Salyin Battle Banner
Shard of Archstone
Kafa Press
Magic Banana
Crystal of Insanity
Sulfuron Slammer Self CC-breaker akin to the old Skull of Impending Doom trick from Vanilla as popularized by Swifty.
Shadowberry Self-healing. Usable in combat, no shared cooldowns.
Glowing Honeycomb Self-healing. usable in combat, no shared cooldowns. Overwrites food buff Unique-5
Engineering:
Frost Grenade
Big Daddy/Cobalt Frag Bomb
High-Powered Bolt Gun (Ranged interrupt and some damage. Requires Obsidium Bolts{consumed})
Discombobulator Ray (Instant Ranged 20% slow. May or may not still dismount) +No skill required
Goblin Dragon Gun, Mark II (frontal Cone, potential unstealther) +No skill required
Ice Deflector and Flame Deflector (No skill required, can absorb single ticks of some DoTs, useful at lower levels) +No skill required
Ornate Spyglass For scouting purposes +No skill required
Stealthman 54 Stealth "potion". Cannot move, cannot be used in combat, 10 min cooldown. +No skill required
Goblin Glider Kit Controllable Slow Fall. 2 min duration, 3 min cooldown. +No skill required
Tailoring:
Frostweave and/or Embersilk Net Embersilk does damage in addition to rooting, but is dispellable as a magic effect (and as such can be prevented with effects like Cloak of Shadows and Antimagic Shell)
Alchemy:
Darkwater Potion/Draenic Swiftness Potion (70% run speed, 200% swim speed)
Living Action Potion/Draenic Living Action Potion(Stun/slow break and 3 sec immunity)
Free Action Potion (Stun/slow immunity, does not break existing effects, 5 sec minimum duration, reduced by level above 60)
Purification Potion (Can remove 1 Curse, Poison and Disease)
Restorative Potion (Same as above, but similar in effect to old Abolish Poison. Has fail chance, be aware)
Elixir of Camouflage (Prevents tracking from hunter/druid/Worg Steak)
Potion of Deepholm (Escape mechanism. Cannot be used in combat)
Mighty Rage Potion/Pure Rage Potion Good for a decent boost of rage in an unexpected kill window
Leatherworking:
Drums of Fury(Bloodlust in a bag) +No skill required
Drums of Speed (30% movement speed, 1.5 min cd)
Drums of Panic (.5sec cast 2s duration AoE fear)
Consumable:
Strange Glowing Mushroom (20 max, 25% max hp and class primary resource over 10 secs) Doesn't work above level 99
Faintly Glowing Herb (10 max, self-heal consumable, 300k heal over 10s) Doesn't work above level 99
Scuttler's Shell (10 max, reduced mob aggro range)
Condensed Jademist (5 max, Vanish)
Falling Flame (50 charges, 1 hr cooldown, escape-best used w/ slow fall or glider)
Bubbling Pi'jiu Brew (10 max)-Minimal use to a rogue or kitty
Misty Pi'jiu Brew (10 max)-Minimal use to a rogue or kitty
Thick Pi'jiu Brew (10 max) -Minimal use to a rogue or kitty
Cracked Talisman (3 charges, 20 second self bubble) Does not work at 100
Cursed Talisman (50 charges, 20 second self bubble, 5 min cooldown) Does not work at 100
Bag of Kafa Beans/Oddly Shaped Horn/Tuft of Yak Fur (3 charges, 300% speed land mount @ 20sec duration)
Feverbite Egg Sac (3 charges, 30 min cooldown, Spawns 6 spiders to eat AoE CC cap)
Redcap Toadstool (Guaranteed single poison removal, on former Healthstone cooldown. Sporeggar rep required)
Noggenfogger Elixir.
Blackened Worg Steak (Food buff, grants Track Humanoids)
Buffs:
Drums of Forgotten Kings: Kings -1%
Runescroll of Fortitude I, II, III: Fort -4/3/2%
If you guys come across anything else of great use, let me know and I'll toss them on the list.